# The spectral function of Mott-insulating Hubbard ladders: From fractionalized excitations to coherent quasi-particles

## TL;DR

This paper investigates the spectral function of two-leg Hubbard ladders using tDMRG, revealing a crossover from spin-charge separation to coherent quasiparticles as inter-leg hopping increases, with detailed analysis of excitation dynamics.

## Contribution

It provides a detailed numerical study of spectral evolution in Hubbard ladders, showing the transition from Mott insulator to band insulator and identifying quasiparticle formation mechanisms.

## Key findings

- Spectral features show spin-charge separation and bound states.
- Increasing inter-leg hopping merges spinon and holon into a quasiparticle band.
- Crossover from Mott insulator to band insulator with a single minimum at $k_x=\pi$.

## Abstract

We study the spectral function of two-leg Hubbard ladders with the time-dependent density matrix renormalization group method (tDMRG). The high-resolution spectrum displays features of spin-charge separation and a scattering continuum of excitations with coherent bands of bound states `leaking' from it. As the inter-leg hopping is increased, the continuum in the bonding channel moves to higher energies and spinon and holon branches merge into a single coherent quasi-particle band. Simultaneously, the spectrum undergoes a crossover from a regime with two minima at incommensurate values of $k_x$ (a Mott insulator), to one with a single minimum at $k_x=\pi$ (a band insulator). We identify the presence of a continuum of scattering states consisting of a triplon and a polaron. We analyze the processes leading to quasiparticle formation by studying the time evolution of charge and spin degrees of freedom in real space after the hole is created. At short times, incoherent holons and spinons are emitted but after a characteristic time $\tau$ charge and spin form polarons that propagate coherently.